Not Applicable
Cast stretch films are high clarity films utilized to protect and unitize manufactured goods or items for transport and storage. Such films with commercial applicability have been produced from ethylene polymers and copolymers. It is highly desirable for such polymers to have excellent processability, i.e., they can be processed into cast stretch film at a faster throughput rate with lower energy requirements and with reduced melt flow perturbations. This has previously required the use of ethylene/xcex1-olefin copolymers with a high shear rate response (where shear response is defined as the high load melt index divided by the melt index of the polymer). It is known to those skilled in the art, that, for such polymers, high melt flow ratio (MFR) values are indicative of a relatively broad molecular weight distribution. Typical Ziegler catalyzed linear low density polyethylene has been used in this application because of its relatively broad molecular weight distribution.
While it is important that polymers used for cast stretch film applications process easily, it is also highly desirable for such films to exhibit high impact strength and on-pallet puncture. For traditional Ziegler Natta catalyzed linear ethylene/xcex1-olefin polymers, increasing impact and puncture strength is often accomplished by narrowing the molecular weight distribution and/or increasing the molecular weight.
U.S. Pat. Nos. 5,231,151 and 5,487,938 (Spencer et al.) teach that linear low density polyethylene (LLDPE) which has a high density fraction, as measured by ATREF, of higher than 17% and a molecular weight distribution (MWD) of less than about 3.6, are particularly effective in making cast films used in pallet wrapping applications.
U.S. Pat. No. 4,243,619 (Fraser et al.) describes a process for making film from a narrow molecular weight distribution polymer compositions which are ethylene/xcex1-olefin copolymers prepared by a Ziegler catalyst useful for stretch film applications and exhibiting good optical and mechanical properties.
U.S. Pat. No. 5,175,049 (Huff et al.) describes the composition of a wrap-packaging cling film which is a film laminate comprising at least two layers. The outer layer comprises a very low density ethylene/xcex1-olefin copolymer (LVLDPE) and at least one of the layers comprises a linear low density ethylene/xcex1-olefin copolymer (LLDPE). These compositions exhibited differential cling properties suitable for wrap packaging.
It is also well known that narrow molecular weight distribution linear polymers possess low melt elasticity causing problems in melt fabrication. Such polymers also experience significant surface melt fracture at relatively low extrusion rates thereby processing unacceptably and causing surface irregularities in the finished product. These irregularities can present difficulties in, for instance, electronic reading of bar codes on packaged material. Thus there remains a difficulty in producing resins which are able to exhibit good processability, while exhibiting improved, on-pallet puncture, dart impact and stretchability and yielding film with few surface irregularities.
The relatively recent introduction of metallocene-based catalysts for ethylene/xcex1-olefin copolymerization has resulted in the production of new ethylene interpolymers. The term xe2x80x9cinterpolymerxe2x80x9d is used herein to indicate a polymer wherein at least two different monomers are polymerized to make the interpolymer. This includes copolymers, terpolymers, etc.
These metallocene catalysts include the bis(cyclopentadienyl)-catalyst systems and the mono(cyclopentadienyl) Constrained Geometry catalyst systems. Such constrained geometry metal complexes and methods for their preparation are disclosed in U.S. application Ser. No. 545,403, filed Jul. 3, 1990 (EP-A-416,815); U.S. application Ser. No. 547,718, filed Jul. 3, 1990 (EP-A-468,651); U.S. application Ser. No. 702,475, filed May 20, 1991 (EP-A-514,828); U.S. application Ser. No. 876,268, filed May 1, 1992, (EP-A-520,732); U.S. application Ser. No. 8,003, filed Jan. 21, 1993 (WO93/19104); U.S. application Ser. No. 08/241,523, (WO95/00526); as well as U.S. Pat. Nos. 5,055,438, 5,057,475, 5,096,867, 5,064,802, and 5,132,380.
In EP-A 418,044, published Mar. 20, 1991 (equivalent to U.S. Ser. No. 07/758,654) and in U.S. Ser. No. 07/758,660 certain cationic derivatives of the foregoing constrained geometry catalysts that are highly useful as olefin polymerization catalysts are disclosed and claimed. In U.S. Ser. No. 720,041, filed Jun. 24, 1991, certain reaction products of the foregoing constrained geometry catalysts with various boranes are disclosed and a method for their preparation taught and claimed. In U.S. Pat. No. 5,453,410 combinations of cationic constrained geometry catalysts with an alumoxane were disclosed as suitable olefin polymerization catalysts. For the teachings contained therein, the aforementioned pending United States Patent applications, issued United States Patents and published European Patent Applications are herein incorporated in their entirety by reference thereto.
The use of metallocene catalyst systems have generated new interpolymers and new requirements for compositions containing these materials. Such polymers are known as homogeneous interpolymers and are characterized by their narrower molecular weight and composition distributions (defined as the weight percent of the polymer molecules having a comonomer content within 50 percent of the median total molar comonomer content) relative to, for example, traditional Ziegler catalyzed heterogeneous polyolefin polymers. Generally blown and cast film made with such polymers are tougher and have better optical properties and heat sealability than film made with Ziegler Natta catalyzed LLDPE. It is known that metallocene LLDPE (see xe2x80x9cEXCEED(trademark) mLLDPE Hexene Copolymersxe2x80x94Structure/Property Relationshipsxe2x80x9d by B. C. Trudell and G. D. Malpass Jr,) offer significant advantages over Ziegler Natta produced LLDPE""s in cast film for pallet wrap applications, particularly improved on-pallet puncture resistance. Such metallocene LLDPE""s however have a significantly poorer processability on the extruder than Ziegler Natta products. The metallocene LLDPE""s known as EXCEED(trademark) (Trademark of Exxon Chemical) have Composition Distribution Branch Index (CBDI) values higher than 65% (see xe2x80x9cEXCEED(trademark) mLLDPE Hexene Copolymersxe2x80x94Structure/Property Relationshipsxe2x80x9d by B. C. Trudell and G. D. Malpass Jr.). Ziegler Natta produced materials have broader composition distributions and have values lower than 65%.
The substantially linear ethylene/xcex1-olefin polymers described in U.S. Pat. No. 5,272,236 (Lai et al.) are also metallocene based homogeneous polymers, as the comonomer is randomly distributed within a given interpolymer molecule and wherein substantially all of the interpolymer molecules have the same ethylene/comonomer ratio within that interpolymer. Such polymers are unique however due to their excellent processability and unique rheological properties and high melt elasticity and resistance to melt fracture. These polymers can be successfully prepared in a continuous polymerization process using the constrained geometry metallocene catalyst systems.
Recent work has examined the properties of blends of metallocene-catalyzed LLDPE and High pressure LDPE or Ziegler catalyzed LLDPE. Such blends typically seek to balance the processability of Ziegler LLDPE with the improved physical properties of metallocene based polymers. Additionally, such blends may lower the cost of production compared to pure metallocene-based polymers due to the higher costs associated with the new and complex metallocene catalyst systems relative to the well-established Ziegler catalyst systems.
Research Disclosure No. 310163 (Anonymous) teaches that blends of Ziegler Natta- and metallocene-catalyzed ethylene copolymers when fabricated into cast films have improved optical, toughness, heat sealability, film blocking and unwind noise properties when compared with metallocene-catalyzed polymer alone. For example a blend comprising 90% of an ethylene hexene metallocene LLDPE (3.0 I2, 0.917 g/cm3) with 10% of ethylene hexene Ziegler Natta catalyzed LLDPE (0.5 I2, 0.921 g/cm3) has increased TD tear resistance and MD ultimate tensile and secant modulus with no other changes in film properties over the 100% blend of the above mentioned metallocene LLDPE.
Research Disclosure No. 37652 (Anonymous) teaches that blends of high pressure polyethylene resins and resins made by single site metallocene catalysts display superior optical and heat sealability properties useful in cast film applications.
Research Disclosure No. 37644 (Anonymous) teaches that blends of traditionally (Ziegler-Natta) catalyzed resins and resins made by single site metallocene catalysts display superior transverse direction tear and machine direction ultimate tensile properties useful in cast film applications.
U.S. Pat. No. 5,376,439 (Hodgson et al.) describes a polymer composition comprising a blend of from about 25 to about 90% by weight of a very low density ethylene polymer having a compositional breadth index greater than about 70% and from about 10 to about 75% by weight of a low to medium density ethylene polymer having a compositional breadth index less than about 70%. The films prepared from these compositions exhibited excellent elongation, tensile and impact properties.
WO 94/25523(Chum et al.) teaches that films having synergistically enhanced physical properties can be made, when the film is a blend of at least one homogeneously branched ethylene/xcex1-olefin interpolymer and a heterogeneously branched ethylene/xcex1-olefin interpolymer. Films made from such formulated compositions have surprisingly good impact and tensile properties, and an especially good combination of modulus and toughness.
U.S. Pat. No. 5,395,471 (Obijeski et al.) teaches the improvement in line speed on extrusion coating equipment for compositions of at least one substantially linear polyethylene alone or in combination with at least one high-pressure ethylene polymer and/or with at least one heterogeneous linear olefin polymer. The new compositions have higher draw down rates, lower neck-in and higher resistance to draw resonance and at the same time maintain or improve on the abuse resistance of the film.
Thus it would be desirable to develop polymers which would allow production of cast stretch films which exhibit good processing characteristics. It would also be desirable to produce cast stretch films with good processing characteristics and good impact strength, puncture and stretchability properties. It would also be desirable to produce cast stretch films which are less sensitive to effects of orientation. Finally, it would also be desirable from a cost point of view to produce such improved cast stretch films from polymers which are blends of metallocene and Ziegler-based catalyst products.
The present invention pertains to interpolymer compositions and cast stretch films made therefrom wherein the interpolymer composition comprises;
(A) a substantially linear ethylene/xcex1-olefin interpolymer present in the composition in an amount of from about 10 to about 100% by weight based on the combined weight of Components A and B; and
(B) a heterogeneous interpolymer present in the composition in an amount of from about 0 to about 90% by weight based on the combined weight of Components A and B; and
wherein said interpolymer composition has an I10/I2 value of less than about 10.0 and is characterized as having a viscosity at 100 rad/s and a relaxation time (xcfx84) which satisfies either of the following relationships;
Log(viscosity at 100 rad/s)xe2x89xa64.43xe2x88x920.8 log(I2)
or
log(xcfx84) greater than xe2x88x921.2xe2x88x921.3*log(I2)
The present invention also pertains to interpolymer compositions and cast stretch films made therefrom wherein the interpolymer composition has two distinct peaks in the ATREF curve; and wherein
T2xe2x88x92T1xe2x89xa625xc2x0 C.
where T2 is the elution temperature of the highest crystallization peak and T1 is the elution temperature of the lowest crystallization peak;
and wherein
Mv1/Mv2xe2x89xa61;
where Mv2 is the viscosity average molecular weight at the peak having the highest elution temperature and Mv1 is the viscosity average molecular weight at the peak having the lowest elution temperature;
and wherein
Mw/Mnxe2x89xa63.
The interpolymers and films of the current invention have a favorable balance of processability and on-pallet properties, and can have a synergistically higher dart impact strength than the Ziegler Natta products and metallocene-based LLDPE""s which do not comprise a substantially linear ethylene/xcex1-olefin interpolymer. The interpolymers of the present invention are similar to polymers prepared from Ziegler Natta catalysts in terms of processability but the on-pallet performance of the cast stretch films of the present invention, including stretchability and puncture resistance is much better. The cast stretch films of the present invention are equivalent to films derived from metallocene LLDPE""s which contain no long chain branching, in terms of their on pallet performance but their processability is much better.